1. Field of the Invention
The present invention relates to a hologram apparatus, such as a hologram recording apparatus, a hologram reproducing apparatus or a hologram recording/reproducing apparatus, for recording and/or reproducing hologram to or from a recording medium of an optical disc, an optical card or the like, and more particularly to a hologram apparatus having a capability of correcting a distortion of a hologram image.
2. Description of the Related Art
In an attempt to record information with high density, attentions are drawn to a hologram technology capable of densely recording two-dimensional data signals. Hologram is characterized by recording a wavefront of light carrying recording information as a three-dimensional change of refractive index in a recording medium formed of a photosensitive material such as a photo-refractive material. For example, there is developed a recording/reproducing apparatus making use of a hologram recording medium as a disc (i.e., hologram disc) (see, Japanese Patent Application Kokai No. H11-311937).
In the hologram recording/reproducing apparatus, reference light is illuminated and converged as a spot on the reflection film through the recording layer. The reference light reflected form the reflection film is scattered to pass through the recording layer. Simultaneously, a signal beam of light carrying information which is to be recorded is passed through the recording layer. In the recording layer, interference of the reflected reference light and the signal light takes places to thereby form an interference pattern, thus enabling hologram recording in the recording layer. Meanwhile, by illuminating reference light to the hologram recording medium and detecting and demodulating the reproduced light from each hologram, information recorded can be reproduced.
However, distortion is caused in the reproduced image due to contraction or expansion of the recording material when hologram recording is performed thereon or aberration of the optical system. The distortion of the reproduced image includes a barrel distortion, a pincushion distortion, a trapezoidal distortion, a defocus (or defocusing) and a positional deviation of the image. Consequently, when reading out a recorded hologram, a distortion occurs in the reproduced image resulting in a mismatch between the light-receiving pixel and an image detection sensor such as a CMOS sensor or a CCD sensor. Due to this, reading is not successful from the region where there is distortion in the image, posing a problem of increased errors in read-out data.
In order to avoid such a problem, it is a practice to perform an over-sampling of data by use of an image detection sensor having pixels greater in the number than those of the spatial light modulator so that correction can be made by image processing when carrying out data reading.
Meanwhile, the conventional aberration corrector, for correcting an aberration (coma aberration, spherical aberration, astigmatic aberration) on the recording medium, could not correct a distortion of hologram image on the photodetector (i.e., reproduced image detection sensor). Furthermore, the image distortion on the reproduced image detection sensor could not be corrected by the conventional aberration corrector because the image distortion is different from an aberration on the recording medium.